Many batteries generate combustible gases during operation. These gases are either vented from the battery container into the atmosphere or recombined within the battery in secondary reactions with the active materials. However, even in batteries which provide for internal recombination of combustible gases, there are operating circumstances in which the recombination mechanism is ineffective and significant volumes of combustible gases are generated. Combustible gases within the head space of a battery may be accidentally ignited and result in an explosion. The damage and injury resulting from such explosions are well documented. Thus, for many years, effective and practical means have been sought for preventing or minimizing explosions in batteries and the hazardous effects thereof.
Combustible gases which are generated within a battery, if not effectively recombined, will eventually create a high internal pressure To alleviate this pressure, these gases must be vented to the atmosphere. Venting is typically accomplished through the use of a simple open vent slot or a one-way relief valve, sometimes referred to as a "burp" valve. During venting of combustible gases, an external source of ignition, such as a flame or spark near the battery vent, can result in an ignition which will propagate back into the battery container and result in an explosion. Improvements in relief valve construction and the development of flame arrestors used in conjunction with vents have considerably decreased the incidence of battery explosions caused by external ignition sources, provided that such protective devices have not been removed or disabled, or that the integrity of the container or cover has not otherwise been breached.
However, should an external source of ignition breach one of the protective devices, or should an ignition occur within the container, the combustible gases in the head space may explode. The concentration of gases, typically a mixture of hydrogen and oxygen in a lead-acid battery, and the relatively large volume of the head space can result in an explosion which will shatter the container, cover or other components. In addition, the explosion will also often carry with it the liquid acid or other hazardous electrolyte from within the container.
Thus, materials and methods for suppressing or minimizing the effects of explosions within batteries have been long sought. Elimination of the open head space, or substantially filling it with a solid material, would virtually eliminate the possibility of an explosion simply because the presence of combustible gases would be eliminated. However, neither alternative is acceptable. An open head space is necessary in virtually all secondary storage batteries. The head space accommodates certain essential battery components, such as plate straps, intercell connectors, or terminals. In addition, in batteries which utilize free liquid electrolyte, sometimes referred to as "flooded" systems, open head space is necessary to accommodate variations in the level of the electrolyte as the battery is cycled, or to provide space for acid movement under extreme conditions of use, such as abusive overcharge. The head space also accommodates movement of the electrolyte level as the battery is tilted in service, such as the ability to operate an automobile on an incline without loss of electrolyte. Thus, due to the need to accommodate certain structural components of the battery and to provide space for electrolyte level fluctuations, the head space in batteries must be maintained.
For years, it has been known to fill the head space in a battery or cell with a porous material to inhibit the explosion of gases within the head space and quench any flame which may be formed, while still allowing the movement of gases and electrolyte through the material. See, for example, Jensen U.S. Pat. No. 2,341,382 issued February, 1944. Other forms of explosion attenuating material have also been proposed. Evjen U.S. Pat. No. 3,846,178 issued Nov. 5, 1974, discloses fitting pieces of closed cell foam into the battery cover and between cells. More recently, commonly-assigned patents provide an explosion attenuating material comprising closely packed pillows made of a foam or a fibrous material such as polypropylene. See Binder et al. U.S. Pat. Nos. 4,751,154 and 4,751,155, issued Jun. 14, 1988, and 4,859,546, issued Aug. 22, 1989. Lining the gas storage chamber of an electrochemical cell has been described in Briggs et al., U.S. Pat. No. 4,004,067 issued Jan. 18, 1977, but only for the purpose of recondensing vaporized electrolyte.
Porous plastic materials have also been used in fuel tanks or similar containers as a means for reducing explosion hazards. See, for example, Allen U.S. Pat. No. 3,561,639 issued February, 1971, Stewart U.S. Pat. No. 3,650,431 issued March, 1977, Funistric, et al. U.S. Pat. No. 4,141,460 issued February 1979, and Sheard et al U.S Pat. No. 4,154,357 issued February, 1979.
A number of factors are believed to have generally inhibited the practical application of explosion attenuation technology in batteries. These include the creation of other hazards, manufacturing difficulty, and detrimental effects on battery performance. Particularly in flooded batteries, the loss of actual open head space volume lessens the space available for electrolyte movement or electrolyte level variations.
It is known that high rate charging or excessive overcharge can result in vigorous gassing in many types of batteries, particularly lead-acid batteries. If the gas bubbles formed in the electrolyte cannot find fairly direct channels to the battery vent openings, electrolyte may be upwardly displaced and overflow through the battery vents. This condition is known as electrolyte pumping. The damaging and hazardous effects of a corrosive electrolyte flowing out of a battery are obvious.
Electrolyte pumping can also occur even where the head space of the battery is filled with a very highly porous material, i.e., a material having a high void volume. For example, an open or closed cell foam material may have a void volume as high as 97 to 99% and, if placed in the head space of a battery, will only occupy about 2 or 3% of the total volume thereof. Nevertheless, in a flooded battery, such a material may readily retain electrolyte and not allow it to drain back into the battery by gravity. Electrolyte so retained in a porous filler material will be readily pumped from the battery under the conditions of vigorous gassing described above.
Further, if a relatively large volume of electrolyte is drawn from the cells through wicking by a porous material in the head space, or if the porous material otherwise retains the electrolyte with which it comes into contact, insufficient electrolyte may remain in the cells for proper electrochemical reaction and operation of the battery.
In the explosion attenuating materials disclosed in the foregoing patents to Binder et al., certain materials which attenuate explosions and quench the flames resulting from the ignition of combustible gases do not perform well in other aspects of battery operation. The violence of an explosion (in terms of the peak pressure developed within the open head space of a battery) can be reduced by filling the head space with certain types of porous materials. The pressure developed during an explosion is reduced as the pore size of the attenuation material is decreased. Unfortunately, as the pore size of the material decreases, the adverse effects of the material on battery performance increase. The smaller the pore size of the material, the greater the propensity of the material to wick up electrolyte, i.e., to retain within the pores electrolyte with which it is wetted.
Absorbed electrolyte cannot drain back into the cell and can result in two serious problems. First, electrolyte retained in the porous material is not readily available for electrochemical reaction, thus diminishing electrical performance of the battery. Second, retained electrolyte will inhibit the flow of gases generated within the battery and, in certain circumstances of operation, result in electrolyte being pumped out of the battery through the vent openings. The use of compacted pieces of explosion attenuating material and the need to fill the head space with such pieces also renders the configurations suggested in the Binder et al. patents difficult to use. The present invention addresses these problems.